﻿Ionization of Metals by Cathode Rays. 379 



It will be observed that the numbers corresponding to 

 states A and B' are by no means constant, and that there 

 is a marked difference between the trend of the numbers in 

 the two rows. In state B' the ionization becomes more 

 nearly equal to that in state B as V increases ; in state A it 

 becomes less nearly equal to that in state B, though the 

 ratio seems to tend to constancy (but not to equality) at the 

 higher values of V. These results seem to confirm com- 

 pletely the view that there is a marked difference between 

 the state A on the one hand and the states B and B' on the 

 other, such as we might expect if in the state A the surface 

 consists of a gas, and in the states B and B' of the metal ; 

 but I am unable to offer any explanation of the variation of 

 the ratio of the ionizations in states B and B' with the speed 

 of the incident rays ; the change does not seem compatible 

 with the view that the lesser ionization in state B' is merely 

 due to a greater opposition to the emergence of the 

 electrons. 



Some experiments were made in a completely different 

 apparatus in which ionization at the surface of a nickel 

 plate was produced, not by cathode rays, but by X-rays, or 

 rather by the secondary cathode rajs excited by X-rays. 

 The experiments were difficult because of the smallness of 

 the effect measured, but it was quite clear that by the passing 

 of a discharge (and change from A to B or B') the ioniza- 

 tion at the surface was considerably decreased. The speed 

 of the secondary cathode rays in these experiments, deduced 

 from the absorbability of the X-rays, was about 50,000 volts. 

 Accordingly, the difference between states A and B persists 

 up to this speed. 



7. Experiments were also made on ihe speed of the 

 electrons liberated at the surface by ihe incident cathode 

 rays ; they consisted of observations of the current flowing 

 from P when the difference of potential between P and B 

 was varied. If U is this potential (counted + when P is + ), 

 then the current is saturated if U is sufficiently large and 

 negative or sufficiently large and positive ; in the latter case 

 none of the electrons liberated at P leave P, in the former 

 case all of them leave P. Let i v be the current from P 

 corresponding to the potential U. Then it is easy to see 

 that, if there is no reflexion of the rays at the surface 



of B, p= . :± ^— .^L is the fraction of the rays liberated at P 



with a speed greater than U. For reasons which will 

 appear presently, / +00 was identified with i , 20 , although the 

 current was not completely saturated at this potential ; it 



